Fiberglass spray lay-up process

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The Fiberglass Spray Lay-up process is similar to the hand lay-up process but the difference comes from the application of the fiber and resin material to the mold. Spray-up is an open-molding composites fabrication process where resin and reinforcements are sprayed onto a mold. The resin and glass may be applied separately or simultaneously "chopped" in a combined stream from a chopper gun. Workers roll out the spray-up to compact the laminate. Wood, foam or other core material may then be added, and a secondary spray-up layer imbeds the core between the laminates. The part is then cured, cooled and removed from the reusable mold.

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[edit] Applications

Applications include making of custom parts in low to medium volume quantities. Bathtubs, swimming pools, boat hulls, storage tanks, duct and air handling equipment, and furniture components are some of the commercial uses of this process.

[edit] Basic Materials

The basic reinforcement material for this process is glass fiber rovings, which are chopped to a length of 10 to 40 mm and then applied on the mold. For improved mechanical properties, a combination of fabric layers and chopped fiber layers is used. The most common material type is E-glass, but carbon and Kevlar rovings can also be used. Continuous strand mat, fabric, and various types of core materials are embedded by hand whenever required. The weight fraction of reinforcement in this process is typically 20 to 40% of the total weight of the part. The most common resin system used for the spray-up process is general purpose or DCDP polyester. Isophthalic polyester and vinylesters are also used in this process. Fast-reacting resins with a pot life of 30 to 40 min are typically used. The resin often contains a significant amount of filler. The most common fillers are calcium carbonate and aluminum trihydrate materials. In filled resin systems, fillers replace some of the reinforcements; 5 to 25% filler is used by weight. Steel, wood, GRP, and other materials are used as mold materials for prototyping purposes. The mold can be a male or female mold. To make shower bathtubs, a male mold is used. In the boating industry, a single-sided female mold made from FRP (fiber-reinforced plastic) is used to make yacht hulls. The outer shell of the mold is stiffened by a wood frame. The mold is made by taking the reversal of a male pattern. Several different hull sizes can be made using the same mold. The length of the mold is shortened or lengthened using inserts and mold secondaries such as windows, air vents, and propeller tunnels.

[edit] Processing Requirements

The processing steps used in the spray-up process are very similar to those in the hand lay-up process. In this process, the release agent is first applied to the mold and then a layer of gel coat is applied. The gel coat is left for 2 hr, until it hardens. Once the gel coat hardens, a spray gun is used to deposit the fiber resin mixture onto the surface of the mold. The spray gun chops the incoming continuous rovings (one or more rovings) to a predetermined length and impels it through the resin/catalyst mixture. Resin/catalyst mixing can take place inside the gun (gun mixing) or just in front of the gun. Gun mixing provides thorough mixing of resin and catalyst inside the gun and is preferred to minimize the health hazard concerns of the operator. In the other type, the catalyst is sprayed through two side nozzles into the resin envelope. Airless spray guns are becoming popular because they provide more controlled spray patterns and reduced emission of volatiles. In an airless system, hydraulic pressure is used to dispense the resin through special nozzles that break up the resin stream into small droplets which then become saturated with the reinforcements. In an air-atomized spray gun system, pressurized air is used to dispense the resin. Once the material is sprayed on the mold, brushes or rollers are used to remove entrapped air as well as to ensure good fiber wetting. Fabric layers or continuous strand mats are added into the laminate, depending on performance requirements. The curing of the resin is done at room temperature. The curing of resin can take 2 to 4 hr, depending on the resin formulation. After curing, the part is demolded and tested for finishing and structural requirements.

[edit] Manufacturing Process

  1. The mold is waxed and polished for easy demolding.
  2. The gel coat is applied to the mold surface and allowed to harden before building any other layer.
  3. The barrier coat is applied to avoid fiber print through the gel coat surface.
  4. The barrier coat is oven cured.
  5. Virgin resin is mixed with fillers such as calcium carbonate or aluminum trihydrate and pumped to a holding tank.
  6. Resin, catalyst, and chopped fibers are sprayed on the mold surface with the help of a hand-held spraygun. The spraygun is moved in a predetermined pattern to create uniform thickness of the laminate.
  7. A roller is used for compaction of sprayed fiber and resin material as well as to create an even and smooth laminate surface. Entrapped air is removed.
  8. Where desirable, wood, foam, or honeycomb cores are embedded into the laminate to create a sandwich structure.
  9. The laminate is cured in an oven.
  10. The part is demolded and sent for finishing work.
  11. Quality control personnel inspect the part for dimensional tolerances, structural soundness, and good surface finish quality, and then approve or reject the part, depending on its passing criteria.

[edit] Advantages

The spray-up process offers the following advantages:

  • It is a very economical process for making small to large parts.
  • It utilizes low-cost tooling as well as low-cost material systems.
  • It is suitable for small- to medium-volume parts.

[edit] Limitations

The following are some of the limitations of the spray-up process:

  • It is not suitable for making parts that have high structural requirements.
  • It is difficult to control the fiber volume fraction as well as the thickness. These parameters highly depend on operator skill.
  • Because of its open mold nature, styrene emission is a concern.
  • The process offers a good surface finish on one side and a rough surface finish on the other side.
  • The process is not suitable for parts where dimensional accuracy and process repeatability are prime concerns. The spray-up process does not provide a good surface finish or dimensional control on both or all the sides of the product.

[edit] Sources

Mazumdar, Sanjay K. Composites Manufacturing: Materials, Products, and Process Engineering. Pp 21-100. Taylor and Francis Group. CRC Press. 2002